War time mystery

This is not for sale for obvious reasons! I found this in a box of my ex-husbands Grandfathers belongings. There was some medals in there also from war, I am assuming WW2, but I honestly don't know. Then I find this thing. It looks to be a necklace? made of rubber that does have metal clasps with a round medallion a little over an inch wide with powder in it. The back is stamped "Poison Inside". I did Google it and came up empty other than there was suicide capsules that looked nothing like this. Any information would be helpful to solve this wartime mystery. I found it in a small box that said Myer's Department Store, Springfield Illinois on it but I do know his Grandfather was from there. Thank you.